Zotero allows you to organize your references in collections, which allow you to easily sort your references by class or assignment.
1. To create a collection, simply click the folder icon in the upper-left corner.
2. Then enter a name for the collection and click OK.
3. To add an item to the collection, click on it in the reference list and drag it onto the folder icon for the collection.
Zotero is free and open source. It is run by a nonprofit who does not mine your data. Learn more about why Zotero is awesome here: https://www.zotero.org/why